I accidentally sent pictures to Martiz that were 3648x2736 and they went through and I had no complaints (until I discovered I had sucked up all my data for the month).
I think they want them 640x480 so they will upload quickly.
I found out what I did when I called them to complain that their site was loading really really slowly and then found out my speed had been dialed down by my crummy ISP because I used up all my data on two shop's worth of photos the night before because the sizes were humongous (accidentally changed a setting when trying to set it to macro mode, hit the wrong button).
Time to build a bigger bridge.