Can't help you much with Illustrator, didn't really study it all that much myself when I was in college.
As for photoshop, changing an images resolution is quite easy.
Just got to Image --> Image Size... or press ALT+CTRL+I and change the resolution numbers from there.
Lets say you got an image you want made into a wallpaper, but its not the right aspect ratio (like say if the wallpaper ratio you want is 16:10 widescreen)
I want to make this image that is 6521x4675 into a wallpaper that is 1680x1050
I would go into the Image Size menu and change either the width or height to the corresponding size I want, in this case, the width to 1680, since changing the height would bring the images width down below 1680.
I would then click the Rectangular Marquee Tool (Or press M), and change the style to Fixed Size
Then change the width and height of the style to 1680x1050 (You may have to put "px" at the end of the numbers as Photoshop defaults these numbers to inches, you can change the defaults by going Edit --> Prefences --> Units and Rulers and change Rulers and Type to pixels), then use the Marquee tool to select the portion of the image I want to be the wallpaper.
Then Crop the image by going Image --> Crop...
Then save your image.
Here's some general rules of thumb to remember when editing images
- It is always better to work with images that are at a higher resolution then your finish product.
- Always scale an image down, never scale up.
- When making changes to an image, make use of layers, this is so that you can turn certian changes off and on as well as using different things if the change you did make didn't work. Some of my works have had over 20 layers.