Limiting picture sizes is common to keep the storage from overloading. Some programs automatically resize to do that, some don't. But, the process isn't that complicated, and after a few tries, it should become second nature. It is much simpler than timing a model A.
Some forums require you to use an URL, which means you have to use a third party setup...this forum is cool as it lets you upload directly from your own computer.
If the picture you want to show is greater than 600Kb (you can right click on general properties to see the size), you need to resize it. If you are running a later version of windows, you can use the application "Paint" that comes with Windows to do that.
Open the picture file by right clicking...you'll see an "open with" option...click on it and choose Paint. Paint will open the picture...Look for "resize" in the upper left-hand corner and click on it. You'll see the full size shown at 100 pixels for both horizontal and vertical. If your original picture is one megabyte (for example) you can change the pixel size to 60 (60/100), both horizontal and vertical will change to keep the picture Horizontal and Vertical relationship the same. Now the size is 600KB.
Click on Paint's unlabeled button in the upper left menu bar to see the options that include "save" or "save as" . If you want to resize the original picture under the original name, simply use "save". If you want to leave the original picture as is, but just want a smaller version to post, click "save as" and rename it.
You can now leave Paint. If you resize but it still comes out too large (user blows it, LOL), you can just resize it again in Paint.
In the forum message box goto "go advanced" and click on the paper clip icon...follow the directions there to find the picture file on your computer and upload it. (I usually place pictures I want to post in a place that is readily found on my desktop).
Then go back to the message body, put the cursor where you want the picture to show, and click on the paper clip icon again, you should see your picture name there, then click on it...it should now be transferred to the message body. You can use the preview to check it out.
If you are using a computer that does not have a version of Paint that resizes picture files (windows 8 on up, as I recall), you'll need another sizing program to go through the same process, almost any photo software will do that.