Adjusting the file size shouldn't be a problem. I resize the dimensions to a good screen size and the use "Save for Web" in Photoshop to compress and save as JPEG. I always make my Web images 632 pixels on the long dimension. Anything between 600 and 800 is goo, in my opinion. The idea is to make the image a size that displays well on a computer monitor and a Web browser. The Photoshop "Save for Web" option allows me to fine-tune the JPEG compression so I get the best compromise between file size and image quality. I usually shoot for a file size of 100-150k.

What image editing software do you have available? Check it for resizing capabilities. And let us know if you have more questions.