For that you can either add "Dynamic image" or "Image" to your form's dataview. Then you need to select the path to your image entity.

Hope this helps.

It could be as simple as just adding a dynamic image widget on your page.

of course you have to make sure that the image object is available from the page, either because the page parameter is the image object or you retrieve it via an association or data source microflow.

The new standard for images is the "Image" widget, visible in João's comment above - this is the replacement for the older static/dynamic images, and will also be compatible with future version of Mendix (10.18+) . For any project currently under construction, I'd advise using this widget instead of the ones suggested above, as this will be long-term viable (supported), while the others are candidates for deprecation.  

Just having the widget will not suffice - you will need to create your own specification of System.Image, associated to you form object (or include the form attributes in your specification); then show a pop up form with this new object as parameter. Only then will you be able to upload/show the image. 

Hope this helps - if it does, please mark as correct.