fix and minify images

doc/workflow/importing/import_projects_from_bitbucket.md

@@ -6,15 +6,15 @@ It takes just a few steps to import your existing Bitbucket projects to GitLab.
 
 * Click on "New project"
 
-![New project in GitLab](bitbucket_importer/bitbucket_import_new_project.jpg)
+![New project in GitLab](bitbucket_importer/bitbucket_import_new_project.png)
 
 * Click on the "Bitbucket" button
 
-![Bitbucket](bitbucket_importer/bitbucket_import_select_bitbucket.jpg)
+![Bitbucket](bitbucket_importer/bitbucket_import_select_bitbucket.png)
 
 * Grant GitLab access to your Bitbucket account
 
-![Grant access](bitbucket_importer/bitbucket_import_grant_access.jpg)
+![Grant access](bitbucket_importer/bitbucket_import_grant_access.png)
 
 * Click on the projects that you'd like to import or "Import all projects"
 
@@ -23,4 +23,4 @@ It takes just a few steps to import your existing Bitbucket projects to GitLab.
 A new GitLab project will be created with your imported data.
 
 ### Note
-Milestones and wiki pages are not imported from Bitbucket.
+Milestones and wiki pages are not imported from Bitbucket.
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