Check the pic you're trying to upload. Are you on windows? If so, right click on the pic, select properties and check the file size. If it's over 4MB, it's not going to load on moparts.
And a 4m picture is poster size , so down size them to upload or host them on your own server
Absolutely, 4 MB is huge. Storage and bandwidth cost money, so keep your pics small if you can, moparts hosts a LOT of pics. If your camera has an email size setting that's perfectly fine for all normal work. Something along the lines of 800x600 if that's one of your choices that works as well.
So if your picture is too big, pull it up in your viewer, fire up snipping tool and take a screenshot of the pic. Save it where you'll know where to find it, give it a relevant name and post that, the size will be fine.
Now, if you're using a Mac, I'm no help at all, sorry.