You can place a button, using the button tool, or any object and set it to be a button by going 'Format > Button Setup...', into the first row of your portal in layout mode and it will appear in all the rows, like your fields.
However to get it to display the related image record you need to either attach a script step to the button, or create a Script if you require any extra actions, above going to the image record, to be carried out and attach that to the button.
The script step to go to the record in the portal row is 'Go To Related Record' and you need to set this up so the settings for 'Get related record from:' uses the relationship the portal is based on, and then 'Show records using layout:' is the layout for the record label. And finally you want to set the 'Show only related records' to 'Match current record only'
You also have the option to show in a new window if you wanted to.
I hope this helps.