based on your tags, it seems you’re using iPhoto on Mac—- no need to use anything else.
in the older versions of iPhoto…..... when you export the photo from iPhoto, you can select the size. keep the aspect ratio the same (the relationship between height and width), and enter smaller numbers than what’s there already. you can guesstimate how much smaller to go based on how ‘too big’ the file is. for ex: if it’s 1 MB and needs to be .3 MB, change the maximum height from 1500 to 500.
in the newer version of iPhoto…..... you can change to a lower resolution—- which is ok if the pictures will appear small. so under ‘size’, from “large” to “medium” or something. or under ‘quality’, change from “maximum” to “medium”. etc.
either way, experiment by trying it—- you’ve got nothing to lose because the photo is still in iPhoto. once it’s on your desktop, click on it and then hit the apple key and the letter “i”—this gives you the “info” on the file—including it’s size. you’ll know if it’s small enough to upload to the website.