it was definitely something with my PNG screen shot, but posting in JPG is not the answer...files are huge.
turns out, if i crop my screen shot and save in photoshop to shrink the file size, i get the above error...never used to get it. this is despite leaving the metadata in the image when i "save for web". weirdest thing. either way, i just uploaded the full screen shot and all is well.
we need someone else with a decent kit of memory and a high BCLK CPU
***got it sorted anyway, just have to fully strip metadata from PNG files for whatever reason