B-Daman Wiki has no image size limits. The height and width values are equal to any number put for the image size. Technically speaking, you could make the file 1000x1000px. Generally however, we would prefer all of our images on infoboxes and templates to be 250px.
I edited it in the Visual editor instead of Soutce mode. The image is very much the same - I renamed it - our file names should be as clear and informative as possible like "CoreRandomBoosterPlus.jpg".
Source editor all the way :) I can't stand the Visual editor, and I'm not a big fan of Classical in general. It kinda reminds me of using Microsoft Word, where you insert a picture and it screws up your whole document. When you're in Source mode, stuff like that doesn't happen. You have full control over the look and formatting of the article at all times. I also find it more comfortable as I code often in HTML, CSS and on forums UBBC, so it's also what feels most natural to me :P Even writing comments like now, I usually use source mode (also because then my browser's spell check works hehe).