I would assume (and its only a quickie thought) that you could edit the language file so the text displays something like:
"The image size is too large. Please make it smaller and try again" instead of the error message that currently pops up
A good instructional topic though - it takes a bit of time granted - would help, and perhaps a message to all members on your forum
Something similar is:
On this forum, users can post images. There are two ways you can do this
If
you have your own webspace, you can "hot-link" directly to the image
from your own webspace by referencing where the image is on your
server. For example, if I wanted to place an image below that was being
hosted on my server, I would do the following:

Which would then produce the image as shown below in this example:

In this case the image ABOVE has been "hot-linked" directly from my own server space
The second, and most common way for users who do not have their own webspace though, would be to FIRSTLY
sign up for a FREE Photobucket Account.
Once signed up, you can then UPLOAD your chosen images to your
Photobucket account, and then reference the image you want to display
within a post like so:

And then you should see the image you want but ONLY after you have posted your reply as shown below:

Ideally, it is best to post images that are not TOO big in size
Generally
speaking I aim to go for say for example 500 pixels WIDE. If you are
using programs to change image proprtions then its fairly easy to do
that. If not, Photobucket allow you to manipulate the image size once
you have uploaded the original
Images that grossly EXCEED this general size will -A) Take up too much forum page space
B) Take too long to download into the page
C) Will be removed or edited
Re-sizing an image posted in Photobucket is done as follows -
Here is how to re-size images uploaded to Photobucket
1. Select the image you want to re-size as shown below, and click Edit
2. It will open a new window showing your Original image. You can now click Re-size
Or you can select one of the preset options
When re-sizing an image always make sure there is a tick in maintain proprtions
Finally save the image as "a copy" of you do not wish to overwrite or replace the original image
Hey presto! Re-sized image as below:

Now at 320x240 pixels
It's fairly easy to get the hang of it, but if you have any questions please let us know
MrTWS
Admin
But please DO NOT hotlink directly - its just a advisory