how to compress a file to half its originial size?

I have 7z and I want to know what settings is best for compressing files.
Currently my attempt at compression gives me a .7z file with the same size as that of the uncompressed file.
Say I have 20mb worth of pictures after compression also the size is 20mb.
where as I see alot of game archives and other files which are bigger in size been compressed to half its size or even less how do they do that?
what settings should i use or what software should i use?
what am I missing?
all my attempts at compression gives me the same size as that of uncompressed file :(
 
The image files you are trying to compress are most probably jpeg or png or some other compressed format themself. It means that the file is already compressed so doing it over and over again won't help. as an example, try compressing a .bmp (which is an uncompressed format) file using 7-zip and you will surely get a very small sized file.

To get a good compression ratio, I use Winrar with the compression mode set to 'best' and 'Create solid archive' enabled. The 'Create solid archive' option only works if you have multiple files selected.

For the best compression ratios and software comparisons, check here
Maximum Compression (lossless data compression software)

Many of the softwares with the best compression ratios have very bad de-compression time/rating. Choose the best one as per your requirement.

for trying out different compression algorithms mentioned use this software.
PeaZip download free RAR 7Z TAR ZIP extraction
 
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