Your image must be a jpeg and the file should be at least 0.5M in size. They don't complain about large files, but they do say that the default display is 1024x1024. However bigger images can be viewed by users. I just loaded a 1600x1600 image and I think that is ideal.
Find the location, then use "Edit" on the blue header of the locality page. This triggers a drop down menu with "Add Photo" as an option. Adding the photo is easy enough, then fill out the detailed form. The nice part about this is that you don't need to type in a bunch of location information, and it ends up where it ought to be in the mindat database.For me, I find it easiest to indicate the field width in mm. I crop my photos square for submission anyway. Then the first menu item is to "calculate the scale bar", which it does nicely given the field width. Be sure to select the right units.
Tom's Computer Info / tom@mmto.org