To qualify to be considered valued, an image must first fit in with the Valued image criteria. Examples of these types of images include high quality images, clear images and ones which are unique to the wiki. When marking an image as valued, please check the following criteria to make sure it can be assessed as that.

An image marked as a valued image must:

Be of a high quality - The image must be of a relatively high quality, and should be clear in a range of sizes. 
Be unique to the wiki - It is important that the image is unique, and will be able to be found on few, or no other websites. This also links in with copyright violations, as if the image is from another online source and is not marked as such, it could be in breach of copyright.
Clearly show the main subject - If the image doesn't make the subject clear, then it may not qualify. If there is a distracting background or other main focus which could be misleading, it must be edited first. 

If you have any concerns about whether your image would qualify, or about any other issue, please contact an admin. Not that when an image is marked as a valued image, the template shown below will be added to the text beneath the file, and the file will be put into the Valued images category.

Here is the template which will be shown on a valued image file page: