This is the tutorial:
0 - Download a file explorer and grant superuser permission (I personally use ES FILE EXPLORER).
1 - Locate this folder: /data/data/com.whatsapp/shared_prefs/
2 - Open the file "com.whatsapp_preferences.xml".
+ Mod image quality (jpg compression level ?)
Look for this line: <int name="image_quality" value="80" />
Change its value (80 by default) to whatever you want (a number between 1 to 99, you shouldn't use 0 or 100 I think).
RECOMMENDED VALUE: 90
Info: at 99 your photos won't have any compression (best quality), but the file size will be MUCH bigger (very close to the original size)
+ Mod image resolution
Look for this line: <int name="image_max_edge" value="1600" />
Change 1600 to the value you want, for example 3200 or 6400 (or bigger).
Info: I think this changes the maximum width allowed for an image. By default, if you want to send a 4K photo (for example) the sent image will be downsized to 1600px. Therefore if you want to send bigger photos change 1600 to 6400 or whatever.
++++
Results (try it by yourself too and tell me):
Here is a 3500x3500 photo, the original file: http://s1.postimg.org/o5rwefjm7/Screenshot_2015_09_16_16_36_20.png
And this is the sent image: http://s27.postimg.org/84yk2ooxv/Screenshot_2015_09_16_16_36_45.png
As you see it keeps the same resolution, you can also increase the quality to 99 to send photos with almost no compression, so the sent image and the original are almost the same, but I don't recommend that value because it takes longer to upload (if you have bad internet like me you won't want to increase the quality to 99), so you should try 90 (which I think is the best quality/compression level).
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srry4myEnglish
0 - Download a file explorer and grant superuser permission (I personally use ES FILE EXPLORER).
1 - Locate this folder: /data/data/com.whatsapp/shared_prefs/
2 - Open the file "com.whatsapp_preferences.xml".
+ Mod image quality (jpg compression level ?)
Look for this line: <int name="image_quality" value="80" />
Change its value (80 by default) to whatever you want (a number between 1 to 99, you shouldn't use 0 or 100 I think).
RECOMMENDED VALUE: 90
Info: at 99 your photos won't have any compression (best quality), but the file size will be MUCH bigger (very close to the original size)
+ Mod image resolution
Look for this line: <int name="image_max_edge" value="1600" />
Change 1600 to the value you want, for example 3200 or 6400 (or bigger).
Info: I think this changes the maximum width allowed for an image. By default, if you want to send a 4K photo (for example) the sent image will be downsized to 1600px. Therefore if you want to send bigger photos change 1600 to 6400 or whatever.
++++
Results (try it by yourself too and tell me):
Here is a 3500x3500 photo, the original file: http://s1.postimg.org/o5rwefjm7/Screenshot_2015_09_16_16_36_20.png
And this is the sent image: http://s27.postimg.org/84yk2ooxv/Screenshot_2015_09_16_16_36_45.png
As you see it keeps the same resolution, you can also increase the quality to 99 to send photos with almost no compression, so the sent image and the original are almost the same, but I don't recommend that value because it takes longer to upload (if you have bad internet like me you won't want to increase the quality to 99), so you should try 90 (which I think is the best quality/compression level).
***
srry4myEnglish