I've posted this before on other forums, but I hope no one minds that I post this again. I would have linked to an old post, but they seem to have scrolled off.
Depending on your level of skill and confidence, you may not need instructions as detailed as these.
First you need a Photobucket account.
Go to Photobucket and open that account.
Find a picture on your hard drive that you want to add to Photobucket. Look at it on your screen.
Click on File, Save as, and take note of what directory it is being saved in. Make sure that it is saving as a jpeg type of file. Write it down if necessary.

Go to PhotoBucket
Go to your album.
It will look like this. Place your cursor in the box UNDER the Add pictures from URL (see the circled item)

Click on BROWSE
You will get a window something like this:

Browse to the folder containing your pictures by double clicking your way to it. It helps if you know ahead of time where they are.
Double click on the photo you want to add.
You will see part of the photo name in the box (see the circled item)

Click on SUBMIT and wait.
When the picture has been added, there will be a message. (see the circled item)

Under the photo you have just added will be up to three lines - URL, TAG and IMG (see the circled item)

You will need the TAG to embed photos into your message. If you click on that box, it will be highlighted. Copy that (CTL-C) and go to your message thread. Paste it in the message where you want it. (CTL-V).
If you don’t see the TAG option, click on ACCOUNT OPTIONS
Make sure there is a dot in the YES circle in front of TAG (see the circled item)

If you want to add multiple pictures, click on the little down arrow beside Submit Multiple Pictures (see the circled item)

There will be a drop down menu allowing you to choose up to 19 pictures.
Browse for each picture as above.

If you discover that a photo is truly too big, after posting, you can go to Photobucket and "edit" the photo.
Click in the edit button and you will see something like this:

If you read carefully, you will see that renaming a file will break links. Don't change the name!!

You can resize (downwards) to your hearts content. Please note that you cannot make a photo larger after you shrink it.
I work in two windows, one with Photobucket open and the other one has the problematic posting open. I usually resize to 75%, then go back to my posting and REFRESH it to see how much smaller it is.
After a while, you develop a sense of whether you need to start reducing to 50% of the original or if 75% is enough.

Hope this helps too.