Hi Sigurþór Halldórsson,
As Ivan noted, the ImageField exists in Sitefinity, but you won't be able to use it in this case. Its only role for now is in the Sitefinity backend when editing image properties.
For your specific requirement you would have to create your own FieldControl and use that when creating the custom field.
That field control would be responsible for displaying the UI for selecting an image (from a library) and saving the value (ID or image URL, whatever you prefer) as the value of the custom field. You need to inherit from the FieldControl class, and your client component should inherit from the FieldControl sitefinity client component. You can do whatever you want with the UI, but make sure to implement the get_value() and set_value() client methods, which will be called by the dialog.
We will consider including such functionality in one of the future releases, but a custom field control is the only way to go for now.
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