You exported the tree, opened it somewhere new, and every photo is gone. The names are there, the dates are there, the little camera icons are there. No pictures.
Nothing went wrong. This is how the format works, and it catches almost everyone the first time.
A GEDCOM is a text file
Open a .ged file in any text editor (or in GED-VIEW's raw GEDCOM Editor, for those comfortable with the format) and it looks like this:
0 @I42@ INDI
1 NAME John /Smith/
1 BIRT
2 DATE 14 MAR 1892
2 PLAC Boston, Suffolk, Massachusetts, USA
1 OBJE
2 FILE C:\Users\Jane\Documents\RootsMagic\Media\smith_john_1892.jpg
2 FORM jpg
2 TITL John Smith, about 1892
That OBJE block is the photo. Or rather, it is a note saying "there is a photo, and it lives at this path on the computer that made this file." The image itself is not in there. GEDCOM was designed in the 1980s to move names and dates between programs, and it has kept that shape ever since. Photos, scans and PDFs stay wherever they were, and the file points at them.
So when the GEDCOM is opened on a different computer, or in a website, the path C:\Users\Jane\Documents\... means nothing. There is no Jane, no Documents folder, no file. The viewer shows a blank thumbnail or nothing at all.
Three situations, three different fixes
Exported from a desktop program on your own computer. RootsMagic, Family Tree Maker, Legacy, Gramps, Family Historian and the rest all keep images in a folder on the disk, and the export writes paths to that folder. The photos are right there. The viewer just needs to be told where the folder is. That is what GED-VIEW's media folder feature is for; the media folder guide covers it step by step.
Exported from a website. Ancestry, MyHeritage and FamilySearch do not put photos into the GEDCOM. Ancestry's export leaves media out entirely, and Ancestry does not offer a bulk download of the gallery from the browser either. The way to get Ancestry photos onto your computer is through a desktop program that syncs with Ancestry, explained below. MyHeritage's GEDCOM export likewise contains the tree without the images. FamilySearch memories are not part of any GEDCOM you can get from FamilySearch.
Someone sent you the file. The paths inside it point at their computer. No software can resolve them. Ask whoever sent it to also send the folder their photos are in. If they exported from a desktop program, that folder is usually easy for them to find, and the paths inside the file tell you its name.
Getting photos out of Ancestry
Ancestry lets two desktop programs sync a tree down to your computer, media included. RootsMagic does it through a feature called TreeShare, and the free RootsMagic Essentials version is enough for this. Family Tree Maker does it through FamilySync. In both cases the program connects to your Ancestry account, downloads the tree, and saves the photos and documents into a media folder on the disk.
Then there are two things: a tree in the desktop program, and a folder of images. Export a GEDCOM from the desktop program, and its OBJE paths point at that folder. Open the GEDCOM in GED-VIEW, pick that folder as the media folder, and the photos are back.
It is more steps than anyone would like. It also means you now hold your own copy of everything, which is a good place to be regardless of which website you use.
GEDCOM 7 and GEDZIP
The newest version of the standard, GEDCOM 7, added a zip package format called GEDZIP (files usually end in .gdz). A GEDZIP is the .ged file plus the media, bundled together, so photos travel with the tree.
It is a real improvement. It is also not widely used yet. Few websites export it, and not every program opens it. If a program can produce a GEDZIP, it is a good way to send a tree and its pictures to a relative in one file. Otherwise, the folder approach is what works today.
What GED-VIEW does with all this
When a GEDCOM is opened in GED-VIEW, it reads every OBJE reference and remembers the file names. With Connect media folder in the file menu you can choose a folder on your computer. GED-VIEW looks through that folder and its subfolders, matches files by name to the references in the GEDCOM, and shows them on the person, in the Gallery, and on any source they are attached to.
Nothing is uploaded. The browser reads the images from your disk to display them. If the browser asks whether to copy files, that is copying into its own cache on your computer, not to a server.
If some images do not match, GED-VIEW lists which ones, so you can see whether they were renamed, moved, or never on this computer at all.
What to do next
If the photos are on this computer, go to the media folder guide and connect them. If they are on Ancestry, set up RootsMagic Essentials or Family Tree Maker and pull them down first. If a relative sent the file, ask them for the folder.
And if the photos really are gone, that happens too. The tree is still intact, and that part is the hard bit to rebuild.