are you sure you need to resize them? or would just modifying the width and height attributes of the img tag work if the aspect was fine.
for example, i am pretty sure you can declare just one of the attributes and the other will scale with the aspect.
i.e. if you have an image that is 1024x768, .. you could use <img src=images/someimage.jpg width=800> and it should display a height of 600. ( i think )
if not, then it is definately time to look at the image library. if this is the case, then my question would be, how do the images get there? is there an upload script? if so, then you could add a function to resize the image at the time of upload.
if not, then it will be more demanding on the server since you will have to check for the image size, and then write logic to resize if the image is over the threshold.