I do not think it's related to Windows 98 but here are some screen captures so you can check if it is the same for you

1) you go to
http://www.boxerworld.com/photo/login.php?login=yes and put your board's username and password.
http://www.boxerworld.com/help/gallery/tutgallery.jpg
After logging in, you get the following page:
http://www.boxerworld.com/help/gallery/tutgallery1.jpg
2) you need to select a category (brindle, fawn, puppy, group or funny. Do not select Members as the picture will always appear in your own gallery too. If your boxer is not a pure breed, please select "Other pets". I have nothing against mutts, I just believe people should not be mislead by some photos thinking it is a pure breed boxer when it isn't) and then to click on "Parcourir" (I am using a French computer) or "Browse" in English. This opens a new window as you can see on the top left of the screen at
http://www.boxerworld.com/help/gallery/tutgallery2.jpg
Go to the folder where your photos are stored and select the one you want to upload by clicking on it twice.
Once it is done, you get the following screen:
http://www.boxerworld.com/help/gallery/tutgallery3.jpg
(On my sample, the photo was on the D drive).
3) Click "Upload/Submit"
It takes a few seconds and then you get the following screen:
http://www.boxerworld.com/help/gallery/tutgallery4.jpg
Put the title (it has to be short) and a description. The description can be long as it will appear below the large version of picture. The title has to be short as it appears on the thumbnails page and there is not much room there.
4) Click "Process files" and you get the following screen:
http://www.boxerworld.com/help/gallery/tutgallery5.jpg
Now if you go to your gallery page, you should see "Image Pending Approval" like at
http://www.boxerworld.com/help/gallery/tutgallery6.jpg
You have to wait until I approve your photo. It is usually approved within a day but it can take several days if I am offline. Keep checking and once it is approved, you see the thumbnail like at
http://www.boxerworld.com/help/gallery/tutgallery7.jpg
Some of the reasons photos aren't approved are: too small, scanned photo that was badly resized/cropped, too many similar photos, filenames starting by a number, special characters like ~, &, %, @ etc in the filenames.
In your case, I did not even get the chance to see the photo pending for approval.
Olly