Your image must be a jpeg and the file should be at least 0.5M in size.
The name of the game is "bigger is better".
I see quality images 3892 by 2498 pixels.
I even see 6000 by 4000 pixels images.
In other words, full camera resolution.
In the past I was timid and cautious and cropped my images square and loaded 1600 by 1600 images. I thought that was great, but I am now am encouraged to go for broke -- which assumes I have quality images.
I have been told that if you are hoping to someday have an image selected for image of the day, you want to give them lots of pixels.
Find the location, then use the "Edit" link on the blue header of the locality page. (If you don't see the "Edit" link, you probably need to log in.). This triggers a drop down menu with "Add Photo" as one of the options. 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.
An important and perhaps tricky business is the field width. For me, I find it easiest to indicate the total field width in mm. At this time, I am cropping 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