Unsupervised image segmentation
Fast answear
Mean Shift LSH which is an upgrade in $O(n)$ of the famous Mean Shift algorithm in $O(n^2)$ well know for its image segmentation ability
Some explanations
If you desire a true unsupervised approach to segment images, use clustering algorithms. The fact is that there is a lot of algorithms with different time complexity and specificity. Take the most famous one, the $K$-Means, it is in $O(n)$ so pretty fast but you have to specify how many cluster you want which is not what you intend by exploring an unknown image without any information about how many shapes are presents in it. Moreover even if you suppose that you know how many shape are present, we may suppose that there shapes are random which is another point where the $K$-Means fail because it is design to find elliptic clusters and NOT random shape ones.
At the opposite we have the Mean Shift that is able to find automatically the number of cluster -- which is useful when you don't know what you're looking for -- with random shapes.
Of course you replace the $K$ parameter of $K$-Means by others Mean Shift parameters which can be tricky to fine tuned but it doesn't exist a tool that allow you to do magic if you're not exercising to do magic.
An advice to image segmentation clustering
Transform your color space from RGB to LUV which is better for euclidean distance.
$K$-Means vs Mean Shift LSH time complexity

Mean Shift : $O(\alpha.n)$
K-Means : $O(\beta.n)$
$\alpha \gt \beta$

Mean Shift LSH is slower but it fits better with your needs. It stay still linear and is also scalable with the mentioned implementation.
PS : My profile picture is an application of the Mean Shift LSH on myself if it can help to figure out how it works.