You are creating your custom page layouts for welcome page, you are deploying it to your site as sandbox solution.
You go to your site that happen to be based on Enterprise Wiki template, edit page, go to “Page layout” you have only 3 options there :
First step to resolve this is to go to Site actions >Site settings > Page layouts and site templates and to add your layouts to available page layouts . (http://site/_Layouts/AreaTemplateSettings.aspx)To be sure, just tick “all layouts”. Click Ok and go back to your page, try to edit it and change page layouts.
Ups, still not there??? Your layouts are still missing?
You go to Page layouts and site templates again and confirm you have done everything correct.
Your may have missed one important step -checking in your page layouts in masterpage gallery. This will happen if you deploy your layouts via sandbox solution – they all checked out and draft.
Check them in and go to page again. Success!!! They can be now chosen for your page