ios image processing related issues

1. Do not use the imageNamed: method for large pictures to avoid the object staying in the memory;
2. When you use the file name and file type to query the file path, the picture file can not be found, because the general picture will have @2x/@3x and other differences for adaptation. Specifications, cannot be found directly;
3. It is best to cache files in a child thread;
4. Objects used in the block do not need to be modified by __block, otherwise the object will be released after the block is executed;