As I said on the title. I just want to know which is better between using image files and drawing vector shapes (or path). I know that using vector is better for appearance but what about performance.
And if this depends on cases. Can anyone explain.
(This question may include WP7, Silverlight, WPF or even in general cases.)
Here is a general answer to compare pros/cons of Bitmap (what I think you mean by "image file") vs. Vector.
Bitmap-based images (gif, tiff, jpeg, png, bmp) are essentially the concept of mapping colours (and other data such as alpha layer) to a pixel grid. Different file formats offer variations of what is supported and levels of compression but this is the high-level concept. The complete map of pixels and data is stored in the file as a matrix/table.
Vector-based images, as you say, are path based. Instead of storing information by pixels, the file format will store geometric points and data.
The pros for bitmaps are:
They usually render faster than a vector. This is because there is minimal computation involved in presenting the image (just take the pixel map and display).
They handle "photographic" content better than a vector.
They are more portable than vector. GIF, JPEG, PNG, BMP are more standard than any vector format (where usually Adobe has the market)
The cons for bitmaps are:
They don't scale without degradation (pixelization)
Manipulation (i.e. resizing, blurring, lighting, etc) of a bitmap is more processor expensive than a vector
The files are usually much larger than vector-based files
The pros for vectors are:
Flexible for scaling and manipulation
Smaller file formats than vector
Ideal for print and animation (i.e. manipulating a shape to produce the animation effect)
The cons for vectors are:
Render time, depending on the complexity of the vector, can be longer
Portability most formats are highly proprietary
Work for "graphic" based images but not useful for photorealism
Hope this helps.