Custom Icons

Unichar makes it simple to include UNICODE symbols in reports. For example the following:

UNICHAR 9013 = UNICHAR(9013)

delivers

 So you can use UNICHAR expressions directly in measures for conditional formatting.

If you want to use specific images, try:

You need to host the icons on a publicly accessible website/hosting service. (for the prupose of this example, I've just referenced images on the web). 

Create a table including the URL to each image and an index column:

Set the Image URL column to type "Image URL"

Create the conditional format measure

Custom Icons =
SWITCH (
    TRUE (),
    [Sum of Sales] > 25800, LOOKUPVALUE ( 'Icon Table'[Image], 'Icon Table'[Index], 1 ),
    [Sum of Sales] < 24000, LOOKUPVALUE ( 'Icon Table'[Image], 'Icon Table'[Index], 2 ),
    LOOKUPVALUE ( 'Icon Table'[Image], 'Icon Table'[Index], 3 )
)

And you will get the following: