What's the best way to save generated images?

I'm writing an app that will generate, store, and display chart images. The idea is this: When a site visitor requests a given chart image, the app will check to see if it already exists. If not, the app will generate and store the necessary image before displaying it.

Towards that end, I have a Chart model with an ImageField:

class Chart(models.Model):
  pattern = models.ForeignKey(Pattern)
  symbols = models.BooleanField()
  rguides = models.BooleanField('row guides')
  cguides = models.BooleanField('column guides')
  hreps = models.SmallIntegerField('horizontal repeats', choices = REPS_CHOICES)
  vreps = models.SmallIntegerField('vertical repeats', choices = REPS_CHOICES)
  image = models.ImageField(upload_to = 'charts/%Y/%m')

So far, so good. From the admin interface, I and view my Charts and upload image files.

Here's my question: What's the best way of saving images that I generate? From what I've read, it seems that my only choice is to create a temporary file, wrap it in a ImageFile object, use it in the construction of a new Chart, call save() on the chart, and delete the temporary file. But given that my image-generation code writes to any file-like object, this strikes me as inefficient. Is there any way to create an ImageFile object and retrieve from it a writable fobj?

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