Use Preview. It’s in your Applications folder. You’ve probably used it for PDF files, as it’s set as the default viewer for those files on all iMacs out of the box.
But it’s also good for images, so drag and drop an image on the Preview icon and it will open in Preview. In the Tools Menu, there is an Adjust Size command, and you can adjust arbitrarily or proportionally. You can also adjust resolution with the same Adjust Size controls, so you can make an image 72 DPI for websites or higher quality for prints.
It’s a good idea to “Save As…” any images you resize in order to keep an original, in case you want to revert changes and go back to an original copy.
And you can also adjust colors and image qualities with Preview. Go to the Adjust Color menu under Tools and see what is possible. You can adjust contrast, saturation, exposure, tint, etc., and even sharpness.
Preview is almost as powerful as iPhoto, so maybe you can do much of what you need in Preview instead of iPhoto.