Tweak preview icons to work in ProjectManager (#276)

Signed-off-by: Pip Potter <[email protected]>

Gem/preview.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:936e6057fd22a0c7ef6c48b66d7495dcab105052eaaf24d11b3baff8cce932dc
-size 29895
+oid sha256:248e3ffe1fc9ffc02afb2ba8914e222a5a5d13ac45a48b98c95ee062e959a94c
+size 4475

preview.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:06e6db9ae8c332ef40921ede4ac1aa5637e9125cf616787d1640809ed8fc565a
-size 117744
+oid sha256:fbf7a7aa55e6b1c8332e9947a507c2ea61a566251fea7edfdc81e31598e5e9e8
+size 98730