If you are running Windows and have an active email client installed, there is an auto photo re-size function available.
Highlight the photo/s you with to re-size.
Right click and choose "send to mail recipient"
A small window will pop up with the option "make my pictures smaller". Click OK.
Now your email client will open along with a new email message started and your re-sized photos will already be attached.
Double click on each attached photo as if to open and view. A box will pop up asking if you wish to "open" or "save".
Choose "save", then save the re-sized photos wherever is convenient, such as desktop.
Close the email message without saving and close your email client.
Now your re-sized photos are saved and ready to be attached to your post.
Pretty much same as above except you choose the new small size in the first window that pops up, "small" is prob best, then click "attach" instead of "OK" as above.
Vista also has a "resize" option accessible under the "file" menu at the top. Highlight your photo, click on "file", select "resize" then choose "small" and finally "Resize and save". This function may only be available when your photo is filed in Windows Live Photo Gallery however.
These steps are generic but usually consistent with the common defaults that most users have, but as with anything having to do with computers, your mileage may vary.