Android :: Managing Large Graphic Set

I'm writing a game for a computer class that I'm currently taking for school. The game has a lot of graphics associated with it (mostly in .png format). There is too much data to simply include it as part of the 'res'. The way i'm currently doing it, is to store all of the graphics in a single zip file, in a folder on the SDCard, e.g., / sdcard/Game/PlayerImages.zip (or whatever). And then i'm using the ZipInputStream to read from those, then using the BitmapFactory to decode the stream and then draw them on the canvas. The problem that I'm having is that this method is terribly slow if I add a lot of images into the zip file. The best way I have found thus far is to name the .png files within the zip numerically, then use a for-loop to jump directly to a specific file, but this is still a very slow way of doing it. I would like to use a single file (like zip) to hold all of the files for several reasons. 1., i'm trying to use a file container so i can simply update graphic 4.png and then push it into the zip file (for example) from a server. 2., because android sees all image files on the sdcard when you open the gallery application. 3., easier to prevent data manipulation by doing an md5/sha-1 checksum on 1 zip file, rather than every time a file is opened if it weren't in a container.