Hi Min,
You will run into problems if you use many images or few images but very large images. Each Android process is allowed only so much memory (on earlier phones, it was 8 MB, nowadays 128 MB is typical, but it depends on the phone and Android version). The Sprite image above, while only being 11 KB on disk, will be roughly 200 KB in memory. This is because for the image to be drawn, it must be uncompressed from the JPG format and represented as a WxH grid of 4-byte pixels (alpha, red, green, blue) that is copied to the screen. To get a sense of how big each picture is to be held in memory, multiply the width and height together and then multiply that by 4. The App Inventor framework will attempt to scale images that are at least 2x the width and height of the screen automatically to reduce the memory load, but that won't protect you indefinitely from OutOfMemoryErrors. So ultimately the number of images you will be allowed to draw at one time will be a function of how big those images are uncompressed and the memory allowed per process on the user's phone.
Evan