You will want to change your marks type to a shape. And then you will drag that field with the text onto the shape shelf. Then you can click the shape shelf of the marks card and change the pallete of shapes there to the one you want to use.
Shapes: Custom Shapes | Tableau Software
Thanks for the tip. Is there any way of auto-assigning an image to a particular data item?
For example, I have a dimension named 'Mos Images' and when I drag this onto the Shapes shelf of the Marks card, it comes up about 66 data items, each with its own label.
I have uploaded all of the specific images that match up to these specific data items, in my tableau repository and have the custom shape palette. I need to know how I can assign this palette so that the images will automatically match up with the correct data item.
When you click 'Assign' shapes, Tableau will automatically assign the shapes based on Alphanumeric order. So you'll need to come up with a naming convention that puts the shapes and labels in the required order. Then it's a one-click process.