How do I specify an image for my property?
Images are in a separate table (media) related to the properties table. The thumbnail of the first related media record is used in the map popover.
What if I don't have an image?
If only some of your records have images, GoMaps will show a "no image" slide for the ones missing images. You can change the placeholder image used by altering the "Default Property Image" in Setup (navigate to the Setup layout in FileMaker Pro, before distributing the mobile file to your users).
If none of your records will have photos, just delete this placeholder image and GoMaps won't attempt to show it. (Another way to remove images from then popover is to comment out the line setting our $sc_ImageURL variable in the script "Compile Map Data" in GoMaps' mobile file.
When / Where are thumbnails created?
On the hosted side, the field Media::Thumbnail is set to an auto-enter calc, so thumbnails are created for you automatically as Media records are populated with images. Change the definition of this auto-enter calc to change the thumbnail size.
On mobile, the field Media::Thumbnail is set using a script trigger upon modification of the Media::MediaContainer field. The script used in the GoMaps mobile file is called "Process Image..." and you can edit the parameter coming into that script to make you thumbnails larger or smaller.
Note that thumbnails are synced DOWN to the mobile GoMaps file (so you don't have to build them on iPad if we already have them on the server) but not synced up to the hosted file: only the MediaContainer field is sent up. And, of course, the auto-enter calc in the hosted file will make that thumbnail as soon as new photos are received.
Thumbnails and sync speed
It is faster to Push your syncs up than to do Round Trips: in Round Trips the iPad's (slower) processor actually makes the thumbnails on the host for new or edited Media records. In a Push, the iPad drops off the MediaContainer and then disconnects: it's a server side script that processes the InBox and thus ends up making the thumbnail. More on InBox Processing.
More general notes about speeding up GoZync can be found here.