problem getting custom button to show

I have a problem that my custom made buttons don't show up online

I'm using Dreamweaver CS5
I downloaded and installed Payment Buttons for Paypal

when in DW creating a new button in the Webassist window, it gives me an option to use a custom button, where i have to add a URL. I put in:
add.jpg
for the location for my image I want to use as a button, when done it shows up in the Webassist window in DW, replacing the standard yellow button, but online it still shows the standard yellow button.

In my DW layout the button both as the standard and my custom shows as two small yellow squares with an H in them followed by a broken images logo, It also did this with the standard yellow button, it worked online with that so I don't mind as long as i know what it is.

I did try the images as a gif instead of a jpg, that did not change anything

Under my saved buttons on PayPals site it shows a box with a cross throu it as the: Your customer's view, instead of my image

I'm not good at code so hopefully it can be solved without having to dive into that
I'm grateful for a hand with my problem

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Update

Thanks for responding fast to solve my problem.
Pasting in the link to the images in the code did the trick.