Systems Management

Are you asking to add a custom field to the service desk ticket layout?
If so, there is no option to add the ability to upload a file/image to a custom field.

Images can be added as part of a ticket comment.

As kevinG says, in a service desk ticket there is no way to add a custom attachment and you must use the mechanism that is already in place, which you cannot make mandatory.

If you are asking from an Asset perspective, then you can add an attachment field into any asset type and you can tick to say that the field is mandatory, so you will not be able to save the asset record without attaching a file. 

I meant as in Tickets, not assets.