I have a serious problem with my OpenCart store and am close to giving up

I have read this whole thread "[1.4.0] Image File Upload Manager" but still cannot get my images working.
I am using v1.4.9.1 and have never used OpenCart before so no previous installs. I have modified the basic template a bit - colours etc. nothing complicated because I really do not know code etc. And all this tinkering was completed before the problem arose.
Last week I created 2 categories and put an image in each one. Everything worked fine. Today I planned to create my full category list. I decided to change one of the existing images so created a new image and used the 'Image Manager' to upload it. It said it had uploaded but it did not show up on the category page. I tried again and nothing. I Deleted the image from the Image/data and Image/cache/data folders. Tried to upload again and I got the title of the image but no actual picture in Image Manager. Yet the image showed up in Image/data and Image/cache/data folders.
Weird I thought so I created a brand new category and uploaded a fresh image but the same thing kept happening. Then when I went back into the category in Admin the Image Manager wasn't even accessible.
My file extensions are .jpg lowercase. My PC and browser are exactly the same as when I last uploaded and image. I am using XP and FireFox. My 'host' has not changed because I am using a Wamp server to learn this software first.
I have just done
parsinsh wrote:he best job is that you must :
1- clear >> system/cache folder
2- clear >> image/cache/data folder
It's a brand new project so I only have about 8 images in total so far. The biggest image is 500px x 500px with a dpi of 96.
And I have turned my PC off and on again more than once

Please, please can anyone tell me where I am going wrong? (In very simple terms please)
Thank you
