If they don't look right, it might be because you didn't resize and/or format them correctly. Usually when this happens the sprites are too small, or blurry, or the movement looks off. A lot of sprites that are added into Essentials need to be doubled in size so that one pixel becomes the size of four pixels. In order to keep the quality of the sprite, there are other options you'd need to check - In GIMP the interpolation needs to be set to none, however it differs from program to program. As for their formatting, most Essentials sprites look like this:
It's a 256x256 image broken up into 16 64x64 squares, each containing a sprite. Note how the top row is the facing down row, the second is the facing left, etc. That's how spritesets work in RMXP. The ones that come with Essentials are different, on rectangles instead, but a nice even 256x256 square is what I would recommend, as it's much easier to work with.
In regards to tilesets, Aki posted a tutorial all about them
right here that should cover pretty much all there is to know.