From iPhoto you can see the resolution of the image in the upper right-hand corner of the program:

Finding the image resolution in iPhoto

This image was taken with an iPhone. 3.5 mb (or 3,584 kb) is much too large to use in a normal post. Normal images for the web should preferably be less than 500 kb. To resize the image for the web go to the “File” menu in the upper right-hand corner and choose “Export”:

Exporting from iPhoto

There will be a dropdown menu:

Export menu in iPhoto

Notice that there are two settings that affect the size of the image: “JPEG Quality” and “Size”. Depending on what the photo is for (does it have text that people are going to need to read or is it just an image) the “JPEG Quality” should be medium to high (probably not maximum) while the size should be either medium or high. Play around with these settings to minimize the size of the image while keeping the image from degrading in quality (you don’t want to post a pixelated image). Choose export to save the image. Verify the image in Preview before uploading.