Open the image in Photoshop, double-click on the layer to make it float. Click on the Add Layer Mask button at the bottom of the layers window, Set your foreground and background color swatches to the default black and white, select the layer mask (to the right of the layer itself), select the gradient tool, then hold down the shift key to constrain the direction to vertical, then drag the gradient tool to create a vertical gradient. You can then adjust the mask as desired. In the file menu use Save for Web and select PNG24 for the format. This will create an image that shifts from opaque to transparent. Save a copy in Photoshop format as well, in case you need to make adjustments later.
Trick for minimizing the dimensions of the file: Open a copy of the stripes file, change the canvas size to a few pixels tall and save it as a JPG. you can use this as an image fill for a rectangle within Hype. Set it to "Scale to fit".
Open the Photoshop file that you used for the gradient. Click on the mask. Look in the Channels palette. You'll see the mask channel. Drag the channel to the dashed circle icon at the bottom (the "load channel as selection" button). You'll see the "marching ants" selection in the main layer. In the Selection menu choose "Selelct inverse) This will flip the selection so that only the pixels with an opacity of 99% and less are now selected. Now go to the Image menu and select Crop. This will trim the image down to only the pixels some degree of transparency. Output this as PNG 24.
In Hype, create a rectangle and fill it with the striped element. Place the trimmed PNG below it so that it touches. The result is an image file that looks the same but is only a fraction of the size if it had been one large PNG.