Their standard size is 350 X 519. Is that a good size for MPlay, or should it be smaller?
According to Endless in another thread, the spatial resolution doesn't matter, it's the file size. The cover art file must be less than 200k. If you are working from a Windows box, that means the size reported in Windows Explorer should be 194k or less.
The posters from impawards often save as jpg's bigger than that. I just right-click, edit, they come up in Paint, and I just hit save. This often brings them down enough in size. Sometimes you have to use Image...Stretch/Skew, and put in something like 90 or 80 percent "stretch" both horizontally and vertically to get the size below the magic 200k. I KNOW there's much better photo editing apps than MS Paint, but Paint is free and comes with every Windows machine.
For content that doesn't have a poster on impawards, I use Media Player Classic to create a "thumbnail" multi-image jpg. I've been using a 4 columns and 2 rows setup with a size of 720 and 99% of the time the result is under the magic 200k. I picked this setup as a tradeoff between the various MPlay art display areas, and I use a default view mode of icons=Large in a folder.cfg in those directories. Works great (for me at least) and though it doesn't come with Windows, it IS free.