I <3 MameWAH.
I mentioned earlier why I'd left MameWAH for a more modern FE - at least one that has been maintained in the past dozen years or so. It was designed for XP. in post win7 environments, there are too many things that don't work and other issues to deal with. I persisted with it longer than most, and that is enough time spent on Mamewah for me. Search this forum for posts over the years and, with a lot of effort and headache, you'll maybe find your solutions.
However, if you find a mostly pain-free way to use MameWAH, please share with us.
Here are four possibly easier solutions: Attract-Mode; Hyperspin; Launchbox; FEEL.
I chose Attract-Mode. It is incredibly easy to setup emulators like MAME with the default layouts/"displays". It is also very open-source and potentially very powerful. If you have some modest skills in object-oriented programming (like with C++ or Python for example), you can do some great things with it.
EDIT: If you still want to persist with MameWAH, you might resize your images. I recall doing this and re-encoding video snaps too. It is fussy and can only handle certain formats.