I know it sounds complicated but it's not that bad once you have done this a few times. I'll step through it using the XP power toy resizer I mentioned above. You will need to download the power toy and install it first. It's a small program that adds another right click option to resize the pictures. It is similar to the feature that resizes pictures for emailing.
Ok so now you have the power toy installed and you want to post a picture from your computer. You need to browse to it and find it on your computer. Place your mouse over it and it should give you a pop up description of the picture. For some reason in my screenshot it did not capture this but in the picture I have circled in red it is 808 KB and is a jpg. See first picture below.
Right click on the image you want to resize and you should see the pop up menu come up with the "resize pictures" option click on that. See picture 2 below circled in red.
When the next pop up box appears you select the actual size you want it resized too. I usually use the small or medium size for posting pictures here to be considerate of those on dial up. The small size should get you below the roughly 100kb limit (actually its like 117-120 kb I think) Check the small circle and then click "OK". This is picture 3 circled in red below.
You may not see the resized image in the windows explorer with the other pictures as it usually adds the resized image at the bottom of the page. However you can go to the top under the "view" menu and then click "refresh". This is picture 4 circled in red below.
This allows you to see the resized image and again if you scroll over the picture you should see it's new size. Again this is not captured on my screenshot but the new resized picture is now 50.9kb. This is picture 5 circled in red below.
Now you are ready to post here. Either create a new post or reply to one. Type in your text and when finished put a check in the little box where it says "check box to attach file (<100Kb)" This is picture 6 circled in red below.
Now you should see a page like picture 7. You want to click on the "browse" button to find the resized picture on your computer. This is much like using windows explorer.
Once you have found the image you want to upload by browsing double click it and it should attach it for uploading. Then you need to click on the "submit" button circled in red in picture 8. You can continue to add pictures until you are finished.
When your done adding pictures be sure to click on the "I'm done click to post" as shown in picture 9 and circled in red.
Now after all these screenshots you will see the picture we resized from approx. 800 kb down to around 50kb. There is little visual difference from the original and most of the detail and colors appear unaffected from the original.
If you are still having trouble drop me an email removing the NOSPAM found in my email address. I hope the visual screenshots will help make it easier than just reading the words of how to do this.