open the image in GIMP.
to change the size go to image--scale image.
change the size to 256 x 256 pixels, or any other size that is power of 2. Be aware that if your image is not square to start with it will probably be distorted when you are done. you can use textures that are not square but they are harder to tile seamlessly. you can also use the rectangular selection tool, select a square section of your image, crop to that selection and then resize your image using the scale tool.
GIMP also has a tool to make your image seamless but it mostly works well with images with little detail. Go to filters--map--make seamless. Eventually you will want to use things such as sharpen, blur, brightness to get the texture that you want.
When you have the size and image that you want go to file--save as. do not click save or you will lose your original image. In the save as options you can change the format to practically anything you can think of. BMP and PCX are used alot, TGA is used alot but can cause some weird artifacts? Texturing can be really in depth, the elusive quant has alot to do with proper texturing and such things as locking your textures on blocks and shaders. or you can get lucky with some simple textures that look good.