If you are using a computer with Windows you should already have Microsoft Paint installed on your computer. This process below changes the actual size of the image, which will reduce the file size. I'm assuming you don't have Photoshop or another more powerful program. If you do, there are also ways to reduce the photo quality & file size while keeping the same dimensions.
1. Go to Start ---> Programs/All Programs ---> Accessories ---> Paint
2. Once the program opens, go to File --> Open and locate the file you want to resize. Click "Open"
3. Go to Image ---> Stretch/Skew
4. In the box that opens, use the first two white text boxes under "Stretch". Enter a number less than 100. (This is the percentage by which the size will be reduced.) Make sure to put the same number in both boxes so that the picture does not become distorted. The smaller the number, the smaller the picture will be.
5. Go to Image ---> Attributes. In the box that opens you can check the dimensions of the picture. Make sure they are within the size limits for posting. If they are not, do not change them. Click "cancel" and repeat step 4 above.
6. When your picture is at an acceptable size, go to File --> Save As. Choose where you want to save your picture, and enter a name. In the "save as type" menu choose JPEG. Hit "Save"
Click the manage attachments button, and when the resulting window opens, just upload the files one after the other. When you're finished, all the pics will be attached to the same post. I think there's a limit of 5 pix per post (or something like that).