OK firstly it is not wise to place images in your database, but instead just place the path to the images in a table and link the images to your page by using a query.
One problem you may be experiencing is the "upload_max_filesize" in your php.ini file
check if your image is larger than the figure in "upload_max_filesize" if it is then PHP is aborting your upload because it wil only upload files less than or equal too the size specified by "upload_max_filesize". In this case just change the figure to whatever you would like and try again.
There are a number of other reasons why this isnt working, is there any chance you could post the form that you are using to upload the file?