The number of pictures that you can store on your built in memory, will vary depending on what settings you are using. Aspect ratio, picture size and picture quality settings all affect the 'size' (number of Kb or even Mb) of the picture.
This is from the user manual
Number of recordable pictures:
"Max. 8 frames (Standard) (DMC-LZ5), max. 6 frames (Fine),(DMC-LZ5), Max. 5 frames (Standard) (DMC-LZ4/DMC-LZ3),max. 3 frames (Fine) (DMC-LZ4/DMC-LZ3), Depends on the remaining capacity of the built-in memory".
An easier way to solve your problem is to insert a 2Gb SDCard (class 6), This is the maximum capacity/speed card that your camera can handle. Ensure that it is a standard SDCard not a SDHC card. It has more than 100 times the storage capacity of your built in memory.This is what they look like. There are a lot of brands , with varying prices. Just make sure you get one with the correct specifications. You may have to format the new card in the camera.
Here is a list of SD card sizes, types and speeds that are suitable to use in your camera.
Hopefully this is of some help