Welcome! Your can post at least 24 photos at a time, in these forum posts. I think dave said therte was no limit.
When you click on "Upload an Image", some text that looks like this will be added to the bottom of your post:
[ lightbox ]2011-11-10/RickCorey/f8ecdc[ / lightbox ]
You won't see the image until you click "Preview".
If you wnat to display them 2-accross or three-accross, add a few spaces before or after each, then delete the "line feeds" or "carriage returns" until 2-3 block sof "light box text" are on the same line. If the text is on the same line, the images will be displayed in the same horizontal row.
By the way - a more permanent way to contribute photos of plants is to add them to the Plant Database. That way, anyone who searches for that plant in the future will see your pictures.
Go to the Home Page where tyhe banner is visible
click "Plants"
search for the plant variety you have a picture of
click "Upload a photo"
If you're not positive of the exact variety, maybe mention that in a caption. Every database contribution is reviewed by Moderators who will confirm your identification.