Each tile is 8x8 pixels. As an example if you take the EA Sports screen it is 40 x 28 tiles, meaning its is 40x8 = 320 x 28x8 = 224 pixels.
When you import a graphic file into TM, make sure you import a image that matches the pixel dimensions (e.g 320x224 as calculated in the example above). Failing to do so can mean when you insert a bigger graphic than expected it can overwrite other required/import bytes that may result in game crashes or odd looking graphics.
This means you will have to resize down (and possible crop) the image you sourced from the internet down to the appropriate size using your favorite graphics editing program, save it as a BMP of PNG (I don't recommend JPG as it can add unwanted artifacts), and then insert it into TM.