Shrink some images, and convert some jpg to png

[tomap]

No description provided.

[tomap]

the build issue come from mozilla/nunjucks#1161
we will need to wait for the fix

[tomap]

It seems yarn install --ignore-engines would solve our issue, or use the lts version of node, as suggested on another pr

[yoshinorin]

@tomap
I think that lts setting at travisCI is better than yarn install --ignore-engines.
I suppose, npm package's owner set engines with intention.

[JLHwung]

I suggest add imagemin-lint-staged in the lint-staged + husky manner:

{
+ "husky": {
+   "hooks": {  
+     "pre-commit": "lint-staged"
+   }
+ },
+ "lint-staged": {
+   "*.{png,jpeg,jpg,gif,svg}": ["imagemin-lint-staged", "git add"]
+ }
}

so that

• all image will be optimized once they are included in git commit (typically for theme author adding new snapshots)
• the image will be only optimized once as they are unchanged
• we don't reply on maintainer to minify them as a task.

[curbengh]

@tomap @yoshinorin I suggest to move travis discussion to a separate issue #798.

[NoahDragon]

As the travis-ci is fixed, can wait to see this change merged. 😉

[tomap]

Build fixed